Assay system for mouse reprogramming factor *
Recently, Yamanaka and his colleagues reported on a novel method for reprogramming of somatic differentiated cells into pluripotent stem cells（iPS cells）by transfecting transcription factor genes. Their new method would solve the problems of immunological rejection and ethics in the case of ES cells, because the somatic cells of each individual could be converted into ES-like pluripotent stem cells and serve for transplantation into the same patients. However there are still many problems to solve. For example, the molecular mechanism and interactions among the introduced genes are unknown as well as whether the reprogramming state using this method would be perfect or not. In this study, the reprogramming of somatic cells was performed by fusion between fibroblasts（tester cells）of female mouse origin that have a GFP gene in one of two X-chromosomes and mouse ES cells. GFP fluorescence negative fibroblasts purified by a cell sorter were used as tester cells. The resulting fused cells showed GFP fluorescence under a fluorescent microscope and GFP protein by western blot analysis. Analysis of the methylation state of the GFP gene in the fused cells revealed that there were more demethylated cytosines in the fused cells than in the tester cells. Moreover, the GFP fluorescence of all of the fused cells disappeared along with their differentiation. These results indicate that X-chromosome reactivation and the reprogramming of somatic cells happen simultaneously, suggesting that both phenomena have a common underlying mechanism. Moreover, reprogramming can be monitored with X-chromosome reactivation through GFP fluorescence. Finally, a question exists as to whether memory for X-chromosome inactivation in the tester cells remained even after reactivation, since GFP fluorescence was completely lost in all fused cells during differentiation.（Accepted on February 28，2008)
